libertas: Set command sequence number later to ensure consistency

Before this patch, the command sequence number is being set before
lbs_queue_cmd() adds the command to the queue. However, lbs_queue_cmd()
sometimes forces commands to queue-jump (e.g. CMD_802_11_WAKEUP_CONFIRM).
It currently does this without considering that sequence numbers might need
adjusting to keep things running in order.

Fix this by setting the sequence number at a later stage, just before
we're actually submitting the command to the hardware. Also fixes a
possible race where seqnum was being modified outside of the driver
lock.

Signed-off-by: Daniel Drake <dsd@laptop.org>
Acked-by: Dan Williams <dcbw@redhat.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>